Scalable orthogonal spin transfer magnetic random access memory devices with reduced write error rates

ABSTRACT

A magnetic device includes a pinned polarizing magnetic layer having a magnetic vector parallel to a plane of the pinned polarizing magnetic layer. The magnetic device also includes a free layer, separated from the polarizing magnetic layer by a first non-magnetic layer, having a magnetization vector with a changeable magnetization direction. The changeable magnetization vector is configured to change to a first state upon application of a first current of a first polarity and to change to a second state upon application of a second current of a second, opposite polarity. The magnetic device also has a reference layer having a magnetic vector perpendicular to the plane of the reference layer and separated from the free layer by a second non-magnetic layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/919,466, filed on Jun. 17, 2013, the entire contents of which arehereby incorporated by references in their entirety.

BACKGROUND

Orthogonal spin transfer magnetic random access memory devices offerfast (sub-nanosecond) initiations of the magnetization write process,combined with low energy operation. These devices employ at least twothin magnetic layers whose preferred magnetization directions are notoriented along the same axis (i.e. not collinear). This assures that aspin-transfer torque acts on a switchable magnetic layer (free layer) ofthe device when a write pulse (a voltage or current pulse) is applied toa device.

SUMMARY

In general, one aspect of the subject matter described in thisspecification can be embodied in a magnetic device includes a pinnedpolarizing magnetic layer having a magnetic vector parallel to a planeof the pinned polarizing magnetic layer. The magnetic device alsoincludes a free layer, separated from the polarizing magnetic layer by afirst non-magnetic layer, having a magnetization vector with achangeable magnetization direction. The changeable magnetization vectoris configured to change to a first state upon application of a firstcurrent of a first polarity and to change to a second state uponapplication of a second current of a second, opposite polarity. Themagnetic device also has a reference layer having a magnetic vectorperpendicular to the plane of the reference layer and separated from thefree layer by a second non-magnetic layer. Other implementations of thisaspect include corresponding systems, memory arrays, and apparatuses.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects,implementations, and features described above, further aspects,implementations, and features will become apparent by reference to thefollowing drawings and the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present disclosure will becomemore fully apparent from the following description and appended claims,taken in conjunction with the accompanying drawings. Understanding thatthese drawings depict only several implementations in accordance withthe disclosure and are, therefore, not to be considered limiting of itsscope, the disclosure will be described with additional specificity anddetail through use of the accompanying drawings.

FIG. 1 shows an orthogonal spin transfer magnetic random access memorydevice in accordance with an illustrative implementation.

FIG. 2 shows an orthogonal spin transfer magnetic random access memorydevice with a polarizing magnetic layer that includes a syntheticantiferromagnet layer in accordance with an illustrative implementation.

FIG. 3 shows an orthogonal spin transfer magnetic random access memorydevice with a reference layer that includes a synthetic antiferromagnetlayer in accordance with an illustrative implementation.

FIG. 4 shows the write error rate (WER) as a function of pulse durationfor a pulse amplitude i=I/I_(c0)=2, a current amplitude of twice thethreshold current in accordance with an illustrative implementation.

FIG. 5 shows the write error rate (WER) as a function of pulse durationfor a pulse amplitude i=I/I_(c0)=3 in accordance with an illustrativeimplementation.

FIG. 6 shows the write error rate (WER) as a function of pulse durationfor a pulse amplitude i=I/I_(c0)=5 in accordance with an illustrativeimplementation.

FIG. 7 shows the write error rate (WER) as a function of pulse durationfor a pulse amplitude i=I/I_(c0)=5 with an expanded x (time) axis, 0 to0.25 ns in accordance with an illustrative implementation.

Reference is made to the accompanying drawings throughout the followingdetailed description. In the drawings, similar symbols typicallyidentify similar components, unless context dictates otherwise. Theillustrative implementations described in the detailed description,drawings, and claims are not meant to be limiting. Other implementationsmay be utilized, and other changes may be made, without departing fromthe spirit or scope of the subject matter presented here. It will bereadily understood that the aspects of the present disclosure, asgenerally described herein, and illustrated in the figures, can bearranged, substituted, combined, and designed in a wide variety ofdifferent configurations, all of which are explicitly contemplated andmade part of this disclosure.

DETAILED DESCRIPTION

An orthogonal spin transfer magnetic random access memory devicegenerally consists of a polarizing magnetic layer oriented orthogonallyto a free layer and includes a reference magnetic layer. These magneticlayers are separated by thin non-magnetic metallic or insulating layers.Described herein, are orthogonal spin transfer magnetic random accessmemory devices that can be reduced in lateral size to the nanometerscale, e.g., ˜3 nanometers (nm), while maintaining the followingcharacteristics for a memory devices: long term stability of themagnetic states used to represent data; reduced write error rates forshort (sub nanosecond) write pulses; and reduced interaction betweenmagnetic devices in a densely packed memory array. In some describedimplementations, an orthogonal spin transfer magnetic random accessmemory device uses balanced switching currents such that the switchingcurrents for writing “1” and “0” states are of similar magnitude.Multiple stable states of the free layer can be envisioned that wouldenable one device to store more than one bit of information.

In some implementations, a memory device can be configured such that thewrite current polarity determines the bit state, e.g., a positivepolarity pulse for a sub-nanosecond duration sets the “1” state and anegative polarity pulse for a sub-nanosecond duration sets the “0”state. In one implementation, the memory device shape can have acircular cross-section. In contrast, when the free layer is magnetizedin-plane the free layer must have an asymmetric shape, because magneticshape anisotropy is used to stabilize the magnetic states (i.e. to setenergy barrier separating the “left” and “right” magnetized magneticstates of the free layer.) Each layer is thin enough that it can bereferred to as a plane. Accordingly, a magnetization field that is“in-plane” means a magnetization field that is parallel to the layer. Asan example, the perpendicular polarizer 102 in FIG. 1 is in-plane. Anadvantage of having a device with a circular shape is that circles arefar easier to fabricate than asymmetric shapes.

The orthogonal spin transfer magnetic random access memory devicedisclosed herein can employ a free layer with a large perpendicularmagnetic anisotropy (PMA). The polarizing layer is magnetized in-plane,and thus provides an initial spin-transfer torque perpendicular to thepreferred magnetization direction of the free layer the instant thedevice is energized with a write pulse. The perpendicular anisotropy ofthe free layer is sufficient that the free layer's lateral extend can bejust 3 nm. That is, the free layer can be a thin film that is circularin cross-section and 3 nm in diameter and a few nanometers thick (0.8 to3 nm), while still being substantially stable against random thermalfluctuations. The condition for stability is that the energy barrier toreverse the magnetization direction K be much greater than the thermalenergy k_(B)T, K/k_(B)T>60, where T is the device operating temperature,typically less than 450 K and k_(B) is Boltzmann's constant.

FIG. 1 shows an orthogonal spin transfer magnetic random access memorydevice 100 in accordance with an illustrative implementation. Frombottom to top: there is an in-plane magnetized polarizing layer (P) 102,a non-magnetic layer (NM1-metallic or thin insulator) 104, the freelayer (FL) 106 which has stable (storage) magnetic states orientedperpendicular to the plane of the layer, either up or down, a thininsulating layer 108, also called a tunnel barrier (NM2), and areference layer (RL) 110 that is magnetized perpendicular to the planeof the layer. The in-plane magnetized polarizing layer 102 has amagnetic state that is oriented parallel to the plane of the layer. Thislayer 102 can be pinned, such that the magnetic state remains parallelto the plane of the layer when current or voltage is applied to thedevice.

FIG. 2 shows a device 200 with a polarizing magnetic layer (P) thatincludes a synthetic antiferromagnet (SAF) layer 202. The SAF layer 202has two magnetic layers 204 and 206 that are separated by a non-magneticmetallic later 208. The magnetic layers 204 and 206 interactantiferromagnetically and thus form magnetic states with theirmagnetizations oriented opposite to one-another. One of the magneticlayers 204 (the lower one in FIG. 2—but, in general, the one that isfurthest from the FL 106) can be placed into contact with anantiferromagnetic to pin (i.e. set) the direction of this magnetic layer204. The magnetic layers 204 and 206 can be coupledantiferromagnetically. Then the magnetization direction of the magneticlayer 206 will be antiparallel to the magnetization of magnetic layer204. Note while the polarizer 202 is indicated as being on the bottom ofthe layer stack 200, the stack can in inverted without changing anyessential aspect of this invention disclosure.

FIG. 3 shows a device 300 with a reference magnetic (RL) layer 302 thatconsists of a synthetic antiferromagnet (SAF) layer. The SAF layer 302has two perpendicularly magnetized layers 304 and 306 that are separatedby a non-magnetic metallic later 308. The magnetic layers interactantiferromagnetically and thus form states in which the layermagnetizations are oriented opposite to one-another. One of the magneticlayers 306 (the upper one in FIG. 3—but, in general, the one that isfurthest from the FL) can be put in contact with an antiferromagneticthat can be used to pin (i.e. set) the direction of this magnetic layer306. The antiferromagnetic layer, however, is not needed for aperpendicularly magnetized SAF. That is, the antiferromagnetic layer isnot required to pin the direction of the magnetic layer 306, because themagnetic anisotropy of the layers 302 can be sufficient to fix themagnetization directions of these layers 302.

The layer structures shown in FIGS. 2 and 3 can be used to optimizedevice preference. First, the SAF layers 202 and/or 302 allow themagnetic interactions between these layers 202 and/or 302 and the freelayer 106 to be configured for various performance needs. The SAF RLstructure (the thicknesses of the magnetic and non-magnetic layerscomprising the SAF) can be used to produce a magnetic field that acts onthe free layer 106. This magnetic field can balance the switchingcurrents for anti-parallel (AP) to parallel (P) and P to AP. Forexample, if the switching current is larger for the P to AP transitionthen the magnetic field from the RL SAF can be set to be parallel to FLmagnetization in the AP state. This can be accomplished by making themagnetic layer 304 in proximity to the free layer 106 within the SAF RL302 thinner or of a lower magnetization density material than the othermagnetic layer 306 in the SAF. In this implementation, the amplitude andthe switching current can be significantly the same for switching thedevice from P to AP and also from AP to P. The currents aresignificantly the same when only the current polarity needs to bereversed between switching from P to AP and AP to P. The currentmagnitude can be the same. This enables use of the same driving circuitsfor switching from P to AP and from AP to P. In these implementations,the polarity of the current determines the device state.

As an example of balancing the switching currents, the SAF RL structurecan have a preference for the P state of the device. Accordingly, lesscurrent is needed to switch to the P state compared to switching to theAP state. To balance the current, such that the same amount of currentis needed to switch to either state, the SAF RL layer can be modified invarious different wants. These modifications introduce some asymmetry inthe SAF RL layer. In one implementation, the magnetic layer 306 can bemade thicker. In other implementations, the magnetic layer 304 can bemade thinner and/or can be made of a material with a lower magneticmoment density. Two or more of these changes can be made to a singledevice to balance the switching currents. An initial SAF-RL that wouldhave a preference for the P state of the device consists of magneticlayers 304 and 306 with the same or significantly similar magnetizationdensities but with layer 304 being thicker than layer 306. Each layer,for example could be composed of [0.3 nm Co/0.9 nm Pt] units repeated N1times for layer 304 and N2 times for layer 306 with N1 greater than N2.A modified SAF-RL would have layer 306 thicker than layer 304 or, as inthe [0.3 nm Co/0.9 nm Pt] example given above, N2 would be greater thanN1. In the modified SAF-RL the net magnetic field acting on layer FL 106would be opposite the direction of the magnetization of layer 304 andthus favor the AP state. Another example would have layers withdifferent magnetization densities but with similar thicknesses. Layer304 could be composed of [0.3 nm Co/0.9 nm Pt]×N while layer 306 couldbe composed of [0.5 nm Co/0.8 nm Pt]×N (with “×N” meaning the thatbilayer of Co/Pt is repeated N times). As the magnetization density [0.5nm Co/0.8 nm Pt] is larger than that of [0.3 nm Co/0.9 nm Pt] thismodified SAF-RL would lead to a net field on FL layer 106 that isopposite the magnetization of layer 304 thus favoring the AP state ofthe device. Having the switching currents balanced allows for reducedcomplexity in the driving circuitry.

The devices of various implementations are scalable because the freelayer consists of a thin film material with a large perpendicularmagnetic anisotropy (PMA). The energy barrier that determines thestability of the magnetic storage states is proportional the magneticanisotropy and the volume of the element. The larger the magneticanisotropy the smaller the volume of the element can be. For fixedelement thickness (˜2 nm) this means that the lateral size of theelement can be reduced. Examples of materials with large perpendicularmagnetic anisotropy are FeCoB in contact with MgO (due to an interfacePMA at the FeCoB/MgO interface). Fe/Pt, Co/Ni, Fe/Pd, Fe/Pt, Co/Pt andCoPd multilayers. The hard-disk industry uses CoFeCr alloys as the mediawhich have large PMA. FePt when formed in the L10 phase has among thehighest PMA of any transition metal magnet. For example, with the PMA ofFePt L10 multilayers, elements that are 2 nm in thickness and 3 nm indiameter are thermally stable, i.e. have energy barriers K tomagnetization reversal greater than 60k_(B)T (K>60 times the thermalenergy at room temperature (T=300 K)).

Multiple devices can be integrated into a memory array capable ofstoring multiple bits of data. For example, multiple devices can begrown together on a single chip that can be used as memory to storedata.

Physical Model of Device

The magnetization dynamics of the device of one implementation can bemodeled to a good approximation by considering the spin transfer torquesassociated with the perpendicular polarizer and the reference layer asfollows:

$\frac{\hat{m}}{t} = {{{- \gamma}\; \mu_{0}\hat{m} \times {\overset{\rightarrow}{H}}_{eff}} + {\alpha \; \hat{m} \times \frac{\hat{m}}{t}} + {{\sin (S)}\gamma \; a_{J}\hat{m} \times \left( {\hat{m} \times {\hat{m}}_{P}} \right)} - {{\cos (S)}\gamma \; a_{J}\hat{m} \times \left( {\hat{m} \times {\hat{m}}_{R}} \right)}}$

(Eqn. 1), where m represents the magnetization direction of the freelayer (it is a unit vector in the direction of the free layermagnetization), and α is the damping parameter of the free layer. Theprefactor, α_(J), depends on the current density J, thespin-polarization P of the current density J, and the cosine of theangle between the free and pinned magnetic layers, cos(θ), such thatα₁=Jg(P, cos(θ))/(2eMt), where  is the reduced Planck's constant, g isa function of the spin-polarization P and cos(θ), M is the magnetizationdensity of the free layer, e is the electron charge, and t is thethickness of the free layer. The last two terms are the spin transferfrom the in-plane polarizer (m_(P)) and the perpendicular magnetizedreference layer (m_(R)). tan(S) represents the ratio of the magnitude ofthese two spin transfer torques.

The magnetic energy of the free layer is given by:

U(m)=−K[({circumflex over (n)} _(K) ·m)²+2h·m]  (Eqn. 2)

where h=H_(ext)/H_(K) is the reduced applied field and H_(K) is theanisotropy field given by the relation: K=(½)M_(S)VH_(K). n_(K) is aunit vector in the direction of the magnetic easy axis of the FL, i.e.perpendicular to the layer plane.

And the effective field in Eqn. 1 is derived from Eqn. 2 as follows:

$\begin{matrix}{H_{eff} = {{{- \frac{1}{M_{s}V}}{\nabla_{m}{U(m)}}} = {{H_{k}\left\lbrack {{\left( {{\hat{n}}_{k} \cdot m} \right){\hat{n}}_{k}} + h} \right\rbrack}.}}} & \left( {{Eqn}.\mspace{14mu} 3} \right)\end{matrix}$

Analysis of Eqns. 1 to 3 shows that there is a threshold current for theswitching of the magnetization direction from aligned parallel to thedirection {circumflex over (n)}_(K) to antiparallel to {circumflex over(n)}_(K) and vice-versa. For currents larger than or equal to thisthreshold value, the initial magnetization state is unstable and willeventually decay into the complementary state. The larger the currentabove this threshold value the faster the magnetization decay rate. Fastdevice writing generally requires currents larger than the thresholdvalue. The decay rate is increased in the presence of thermal noise,which can aid the write process.

Threshold values of the current are given by:

$\begin{matrix}{{I_{c\; 0} = {\frac{2\; e}{\hslash}\frac{\alpha}{P}K}},} & \left( {{Eqn}.\mspace{14mu} 4} \right)\end{matrix}$

for g(P, cos(θ))=P, that is the function g is a constant equal to P, thespin polarization of the current. In general I_(c0) will depend on theform of the function g. When g depends on the angle θ there can bedifferent threshold currents for switching from P to AP and AP to P.

In order to describe the device writing process, thermal fluctuations ofthe magnetization must be included in the physical model of the device.This is included by adding a Langevin field to the effective field:

H_(eff)→H_(eff)+H_(th)  (Eqn. 5),

where H_(th) is a Gaussian distributed random magnetic field thatrepresents the thermal noise associated with the switchable layer beingat a temperature T:

<H_(th)>=0

<H _(th,i)(t)H _(th, k)(t′)>=2Dδ _(i,k)δ(t−t′)  (Eqn. 6a and 6b)

The quantity D is given by:

$\begin{matrix}{D = {\frac{\alpha \; k_{B}T}{2\; {K\left( {1 + \alpha^{2}} \right)}} = {\frac{\alpha}{2\left( {1 + \alpha^{2}} \right)\xi}.}}} & {\left( {{Eqn}.\mspace{14mu} 7} \right).}\end{matrix}$

The value ξ introduced as K/k_(B)T=ξ, so that ξ the ratio of the energybarrier to the reversal of the magnetization divided by the thermalenergy. As noted above, ξ is typically larger that 60 for long termstability of the device memory states.

The write error rate of the device can be determined by solving thestochastic differential equations (Eqn. 1 to 7) numerically usingparallel computational methods based in graphical processing unitssimulated in parallel on an NVidia Tesla C2050 graphics card. The largenumber of necessary random numbers were generated by employing acombination of the three-component combined Tausworthe “taus88” 16 andthe 32-bit “Quick and Dirty” LCG17.

FIGS. 4-7 show the results of these calculations and methods to optimizedevice performance. FIG. 4 shows the write error rate (WER) as afunction of pulse duration for a pulse amplitude i=I/I_(c0)=2, a currentamplitude of twice the threshold current. Results are shown fordifferent ratios of the spin transfer torques from the polarizer andreference layer, the parameter S. Larger values of S give a reduced WER.For example, for a 1 ns pulse the WER can be reduced by a factor of 5 byincreasing S from 0 to 9b, where b is π/36.

FIG. 5 shows the write error rate (WER) as a function of pulse durationfor a pulse amplitude i=I/I_(c0)=3.

FIG. 6 shows the write error rate (WER) as a function of pulse durationfor a pulse amplitude i=I/I_(c0)=5.

FIG. 7 shows the write error rate (WER) as a function of pulse durationfor a pulse amplitude i=I/I_(c0)=5 with an expanded x (time) axis, 0 to0.25 ns.

These results show that increasing S can greatly reduce the WER forsub-1 ns pulses. The effect is very significant as the current pulseamplitude is increased. For example, for i=3 the WER can be reduced by afactor of 30 for pulses between 0.1 and 1 ns when S is increased from 0to 12b. For i=5 the WER is reduced by a factor of 10,000 for the sametime range when S is increased from 0 to 12b. These are very significanteffects and reducing the WER at these time scales can reduce a memoryarray's complexity (for example, less error correction circuitry isneeded in the memory array).

Having a free layer that is perpendicular to the plane of the device, asshown in FIGS. 1-3, eliminates or greatly reduces the precessionalswitching of device's state that can occur at high currents. In someorthogonal spin transfer magnetic random access memory devices, when asufficiently large current or voltage is applied, the spin-transfertorque of the polarizer can lead to a torque that rotates the free layermagnetization out of the free layer's plane. This can then produce ademagnetization field perpendicular to the free layer's plane aroundwhich the free layer magnetization precesses. Precession can occur forboth signs of the current, since in both cases the free layermagnetization is tilted out of the free layer's plane. The free layermagnetization rotation about its demagnetizing field will result in aswitching probability of the free layer's magnetic state that is anonmonotonic function of the pulse amplitude or duration, since if thepulse ends after the free layer magnetization finishes a full rotation(i.e., a 2π rotation), the probability of switching will be reduced.Eliminating or greatly reducing precessional switching means themagnetic state of the free layer will not precess. Accordingly, thetiming of the current pulse is not required to be as exact as comparedto the timing requirements when precessional switching can occur, e.g.,when the magnetization vector of the free layer is parallel to the planeof the free layer.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular implementations of particularinventions. Certain features described in this specification in thecontext of separate implementations can also be implemented incombination in a single implementation. Conversely, various featuresdescribed in the context of a single implementation can also beimplemented in multiple implementations separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Thus, particular implementations of the subject matter havebeen described. Other implementations are within the scope of thefollowing claims.

What is claimed is:
 1. A magnetic device comprising: a pinned polarizingmagnetic layer having a magnetic vector parallel to a plane of thepinned polarizing magnetic layer; a free layer, separated from thepolarizing magnetic layer by a first non-magnetic layer, having amagnetization vector with a changeable magnetization direction, whereinthe changeable magnetization vector has a state that changes to a firststate upon application of a first current of a first polarity and thatchanges to a second state upon application of a second current of asecond, opposite polarity; and a reference layer having a magneticvector perpendicular to the plane of the reference layer and separatedfrom the free layer by a second non-magnetic layer, wherein thereference layer and the pinned polarizing magnetic layer balances anamplitude of current required to change the changeable magnetizationvector state such that the amplitude of the first current issignificantly the same as the amplitude of the second current, andwherein the reference layer comprises: a first magnetic layer that has afirst magnetization vector oriented perpendicular to the first magneticlayer; and a second magnetic layer, separated from the first magneticlayer by a reference layer non-magnetic layer, that has a secondmagnetization vector oriented perpendicular to the second magnetic layerand opposite of the first magnetization vector, wherein thicknesses ofthe first magnetic layer and the second magnetic layer are different andbalance the amplitude of current required to change the changeablemagnetization vector state.
 2. The magnetic device of claim 1, whereinthe magnetic device is between 3 nanometers and 10 nanometers in height.3. The magnetic device of claim 1, wherein the changeable magnetizationvector changes to the first state based upon a pulse of the firstcurrent between 0.1 nanoseconds and 0.8 nanoseconds.
 4. The magneticdevice of claim 3, wherein the changeable magnetization vector changesto the second state based upon a pulse of the second current between 0.1nanoseconds and 0.8 nanoseconds.
 5. The magnetic device of claim 4,wherein the pinned polarizing magnetic layer has a first spin torque andthe reference layer has a second spin torque, and wherein a ratio of thefirst spin torque and the second spin torque reduces write error rates.6. The magnetic device of claim 5, wherein the ratio of the first spintorque and the second spin is between 0.36 and 1.8.
 7. The magneticdevice of claim 1, wherein the magnetic device has a circular crosssection.
 8. The magnetic device of claim 1, wherein the first magneticlayer comprises a first number of Co and Pt layers.
 9. The magneticdevice of claim 8, wherein the second magnetic layer comprises a seconddifferent number of Co and Pt layers.
 10. The magnetic device of claim1, wherein the first magnetic layer and the second magnetic layer havedifferent magnetic moment densities.
 11. A memory array comprising: aplurality of memory cells, wherein each memory cell comprises: a pinnedpolarizing magnetic layer having a magnetic vector parallel to a planeof the pinned polarizing magnetic layer; a free layer, separated fromthe polarizing magnetic layer by a first non-magnetic layer, having amagnetization vector with a changeable magnetization direction, whereinthe changeable magnetization vector changes to a first state uponapplication of a first current of a first polarity and to change to asecond state upon application of a second current of a second, oppositepolarity; and a reference layer having a magnetic vector perpendicularto the plane of the reference layer and separated from the free layer bya second non-magnetic layer, wherein the reference layer and the pinnedpolarizing magnetic layer balances an amplitude of current to change thechangeable magnetization vector state such that the amplitude of thefirst current is the same as the amplitude of the second current, andwherein the reference layer comprises: a first magnetic layer has afirst magnetization vector oriented perpendicular to the first magneticlayer; and a second magnetic layer, separated from the first magneticlayer by a reference layer non-magnetic layer, has a secondmagnetization vector oriented perpendicular to the second magnetic layerand opposite of the first magnetization vector, wherein thicknesses ofthe first magnetic layer and the second magnetic layer are different andbalance the amplitude of current required to change the changeablemagnetization vector state.
 12. The memory array of claim 11, whereineach of the memory cells is between 3 nanometers and 10 nanometers inheight.
 13. The memory array of claim 11, wherein the changeablemagnetization vector of each of the memory cells changes to the firststate based upon a pulse of the first current between 0.1 nanosecondsand 0.8 nanoseconds.
 14. The memory array of claim 13, wherein thechangeable magnetization vector of each of the memory cells changes tothe second state based upon a pulse of the second current between 0.1nanoseconds and 0.8 nanoseconds.
 15. The memory array of claim 14,wherein the pinned polarizing magnetic layer of each of the memory cellshas a first spin torque and the reference layer of each of the memorycells has a second spin torque, and wherein a ratio of the first spintorque and the second spin torque reduces write error rates.
 16. Thememory array of claim 15, wherein the ratio of the first spin torque andthe second spin is between 0.36 and 1.8.
 17. The memory array of claim16, wherein each of the memory cells has a circular cross section. 18.The magnetic array of claim 11, wherein the first magnetic layercomprises a first number of Co and Pt layers.
 19. The magnetic array ofclaim 18, wherein the second magnetic layer comprises a second differentnumber of Co and Pt layers.
 20. The magnetic array of claim 11, whereinthe first magnetic layer and the second magnetic layer have differentmagnetic moment densities.